Theory 1:
They make noise, but not in public.

Theory 2:
They are mocking us.

Theory 3:
They are being smart-arses and are using hands-free cell phones

Theory 4:
They speak at exactly the same time we do, so therefore we do not hear.

Theory 5:
High frequency whistles. A sound known to aggrivate penguins ( which are rarely seen socialising with giraffes ).

Theory 6:
They have nothing good to say about anyone, therefore they say nothing at all.

Theory 7:
They want say goodbye, which is the hardest word to say.